1. Field of the Invention
The present invention relates to a power-driven focusing apparatus known as a power focusing device which drives a motor by a manually actuated switch to drive a photographing lens to an in-focus position.
2. Related Background Art
In prior art focusing for photographing, a rotational movement of a ring-shaped focusing ring is mechanically translated to a linear movement by a helicoid screw mechanism or a cam mechanism to drive a lens, and manipulation feeling as a human operated tool is very good.
However, as auto-focusing cameras have increased recently, the focusing of the photographing lens by a motor drive has become common, and a so-called power focusing mechanism which turns on or off an electrical switch in accordance with the actuation of a manually manipulated member such as a push button or a sea-saw lever and controls the rotation of a motor in accordance with the switch signal to drive the focusing lens has been widely adopted.
However, in such a power focusing mechanism, since structures of the switch and the manually manipulated member are different from that of the ring-shaped focusing ring in the prior art mechanical focusing device, the same good manipulation feeling as that in the prior art device is not attained and this makes the fine adjustment of focusing very difficult.
As an approach thereto, it has been proposed to use a rotary dial or ring as the manipulation member, detect the rotation thereof by a rotary encoder and control the motor in accordance with the detected rotation. This method provides good manipulation feeling but the mechanism to detect the rotation is complex and precise and software to control the motor is also complex. As a result, it is disadvantageous in cost and manufacture.
It has also been proposed to use a power focusing device which uses a push button of a simple structure to detect an actuation time of the switch, control the motor in accordance with the actuation time of the switch so that the motor is driven during the actuation of the switch if the actuation time is long while the motor drive is controlled to a constant amount if the actuation time is shorter than a predetermined period. (Japanese Laid-Open Patent Application No. 59-64816) In this method, the fine adjustment of the focusing is attained relatively easily, but it is necessary to intermittently manipulate the manipulation member many times during the fine adjustment stage of the focusing, and quick focusing operation is hard to attain.